1. Field of the Invention
The present invention relates to a tire for heavy load use capable of improving the durability of bead portions.
2. Description of the Prior Art
A tire for heavy load use is filled with high air pressure and is used under demanding conditions of largely applied load. Bead portions that bear a large amount of load are thus firmly reinforced, have a large thickness and also have an extremely large weight. For reducing weights of such a tire for heavy load use, it has conventionally been suggested for a tire of bead structure as illustrated in FIG. 9 in which ply turn-up portions b2 that are wound around bead cores c by approximately a full round on both end portions of a main portion b1 of a carcass ply b extending between the bead cores c, c in JP-A 11-321244(1999) and JP-A 2000-219016.
In such a bead structure, outer ends of the turn-up portions b2 are formed to be proximate to peripheries of the bead cores c that are exposed to small strains at the time of performing running under load condition of the tires, so that they are hardly affected through strains during running. Accordingly, damages such as loosing of cords originating at the outer ends of the turn-up portions b2 hardly occur. Moreover, since the length of the turn-up portions b2 of the carcass ply b can be made short, it will also contribute to weight saving of the tire weight.
However, since the above bead structure is arranged in that the turn-up portions b2 are short and in that the degree of bending thereof is large, the bending of the turn-up portions b2 try to return to the original shape in, for instance, the course of raw tire forming. As a result, air holes are apt to be formed between the turn-up portions and the bead core c so that deficient moldings such as air residues are apt to occur. Such deficient moldings present disadvantages in that engaging force proximate of the bead cores 5 of the carcass 6 is reduced so as to induce blow-by phenomena.